Multiple input proximity detector and touchpad system

ABSTRACT

A touchpad is formed of an electrically insulating membrane (10) with a first series of spaced apart conductors (12) on a first face of membrane (10) and a second series of spaced apart conductors (14) on or proximal thereto, in which there is no electrical contact between the first and second series of conductors (12, 14) Each conductor in the first and second series of conductors is sensitive to the proximity of a finger to modify the capacitance of the proximate conductor to detect the presence of the finger positioned close to that conductor. A scanning system operative to sample one of the conductors in turn from both the first and second series of conductors (12, 14) in order to measure and store a capacitance value associated with that respective conductor. The scanning system is operative to maintain all conductors (12-n, 14-n) at a common potential equal to the potential of the conductor being sampled when the remaining conductors are not actively being sampled by the scanning system.

The present application is a continuation of an application entitled"MULTIPLE INPUT PROXIMITY DETECTOR AND TOUCHPAD SYSTEMS", filed Oct. 3,1996 and assigned Ser. No. 08/718,356, now U.S. Pat. No. 5,844,506 whichis a national application based upon British PCT Application entitled"MULTIPLE INPUT PROXIMITY DETECTOR AND TOUCHPAD SYSTEM", assigned Ser.No. PCT/GB95/00767, filed Apr. 5, 1995, claiming priority to a Britishapplication entitled "MULTIPLE INPUT PROXIMITY DETECTOR AND TOUCHPADSYSTEM", filed May 4, 1994 and assigned Ser. No. 9406702.2, all of whichapplications describe and claim inventions made by the present inventor.

The present invention relates to a multiple input proximitydetector/touchpad system which may comprise, for example, a keypadarray, digitising tablet, touchscreen or an electronic mouse which maybe operated through a variable thickness of glass or other dielectricmedium, and more particularly to the design of a multiple inputproximity detector/touchpad system in which a large matrix of keys or alarge touch sensitive area may be formed using the superposition of, forexample, orthogonally arranged conducting elements. The conductingelements, and the electronic scanning system to service the conductingelements are particularly arranged to obtain optimized sensitivity.

In European Patent No. EP-0185671 there is described a touch operatedkeyboard for attachment to one face of a sheet of glass comprising aplurality of keypads disposed adjacent each other in a desired pattern,together with interrogation means for assessing the condition of thekeypads, indicating when a keypad, or keypads have been operated by auser, and an electronic scanning and processing system for providingmeans for threshold value generation and drift compensation.

The threshold value generation means is operative to determinerepeatedly at predetermined intervals the required capacitance levelassociated with any keypad in order to indicate that that keypad hasbeen operated by a user.

The drift compensation means is operative to offset variations incapacitance caused by varying background conditions.

The present invention is directed towards the construction of a multipleinput proximity detector/touchpad system, which may comprise a keypadarray, digitising tablet, touchscreen or an electronic mouse, whereinthe position of a user's finger or other object touching, or in closeproximity to the "touch sensitive" surface area, hereinafter referred toas a touchpad, is determined by means of the capacitive effect of thatfinger on multiple conductor elements (hereinafter referred to as akeystroke), and to the optimisation of sensitivity of the touchpad,particularly when the touch sensitive area becomes relatively large. Itis intended that throughout the present specification, reference to a"finger" is intended to include any object that would exert sufficientcapacitive influence to be detected by the touchpad.

It should be noted that the activation of a "keypad" or area of thetouchpad can be achieved without pressure on, or even without physicalcontact with, the surface of the touchpad, although in normal mode ofoperation, the user's finger would contact the touchpad surface or asurface associated therewith.

Other known types of touchpad, such as membrane switches having two setsof conductors face to face, require the use of pressure on twoconducting elements at an intersection of those conducting elements.Pairs of conducting elements may be scanned in systematic manner todetermine which, if any, intersection has been pressed. Disadvantages ofthis system are that there are moving parts (eg. the upper surfacepresented to the user's finger) which can therefore be subject todamage, and also that the positioning of the user's finger must coincidewith the conducting element intersection. This method employs a set ofdriver conductors and a set of sensing conductors.

The present invention, however, uses only sensing conductors and has nomoving parts. It can thus be well protected from damage by users by theglass or other dielectric medium covering the touchpad. The electronicscanning of the conducting elements requires connection to only oneelement at a time, and all other elements can be placed in condition toreduce interference when not being scanned. The present inventionfurther permits detection of the user's finger at any point on thetouchpad's active surface, and the electronic scanning mechanism couldbe arranged to assign predetermined areas of the touchpad to beinterpreted as discrete keypads, or "boxes".

Of fundamental importance to such a touchpad system is the sensitivityof the apparatus to the proximity of a finger when compared with normalvariations in capacitance. This ensures reliable indication of anintentional "keystroke" as previously described, and the determinationwith a high degree of accuracy of the position of that finger. Theposition of the finger may be a digital representation of which "box" orpredetermined area of the touchpad has been activated from a set ofpossible boxes, or predetermined areas, or alternatively an analoguerepresentation of the position by, for example, x-y coordinates.

The present invention is further directed towards the achievement ofthis required sensitivity, by a number of alternative embodiments whichmay be used separately or in conjunction with one another.

Applications of such a touchpad are many and diverse, for example:

as a touchscreen interface for a computer system the keyboard beinglocated immediately in front of a display unit which may be, forexample, a cathode ray tube or liquid crystal display;

a cash till keypad, where there would normally be many buttons forspecific or different types of merchandise (the present invention isparticularly suited to this application where the till operator islikely to have dirty or greasy hands, since the invention can provide asmooth glass for the keypad surface which is easily wiped clean);

as an equivalent to a "mouse" input device to a computer system wherethe screen cursor is moved by moving one's finger across the surface ofa touchpad;

as a standard layout keyboard for use in a hostile environment;

as many discrete proximity sensing keys.

In the environment of a cathode ray tube, or other static-orinterference-generating device, it may be necessary to protect thetouchpad from such static by known means, for example a transparentearthing shield. Alternatively, an actively driven back plane may beused.

In a general aspect the present invention provides a multiple inputproximity detector in which the juxtaposition of two or more independentsensor inputs are used to determine the proximity of a finger, suchdetection only being accepted as valid when all the sensor inputsindicate a valid detection, where such inputs may be juxtaposed next toa range of other inputs in unique combination such that when any onecombination gives a true detection for all its individual inputs, aunique valid detection is determined.

According to the present invention there is provided a touchpadcomprising an electrically insulating membrane with a first series ofspaced apart conductors on a first face of the membrane and a secondseries of spaced apart conductors on or proximal thereto, in which thereis no electrical contact between the first and second series ofconductors, each conductor in said series being sensitive to theproximity of a finger to modify the capacitance of said conductor todetect the presence of said finger positioned close to that conductor.Preferably the first and second conductors comprise fine wirespreferably of a size between 10 to 25 microns to be substantiallyinvisible when the touchpad is used as a touchscreen.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows in plan view a touchpad according to the present invention;

FIGS. 2a, 2b and 2c show in alternative embodiments cross-sectionsthrough the touchpad of FIG. 1, not to scale;

FIGS. 3a and 3b show embodiments of intersection points of twoconducting elements;

FIG. 4 shows in plan view an embodiment of the present inventionsuitable for a large area touchpad with multiple conducting elements;

FIG. 5 shows in cross-section an embodiment of the present invention inwhich connections can be made between conducting elements;

FIG. 6 shows a small part of a touchpad surface;

FIG. 7 shows a part of a touchpad surface indicating an embodiment ofthe invention in which multiple keypad areas are assigned to eachintersection;

FIG. 8 shows schematically an embodiment of scanning apparatus suitablefor use with the touchpad;

FIG. 9 shows a pattern of conductor elements suitable for use in thestyle of a standard typewriter keyboard layout; and

FIG. 10 shows a pattern of conductor elements demonstrating oneembodiment of a multiplexed touchpad.

With reference to FIGS. 1 and 2a, and according to one embodiment of theinvention, there is provided a thin dielectric film 10 on which isdeposited on one face by an appropriate technique such as screenprinting or similar lithographic process, a pattern of electricallyconducting material forming a first series of parallel conductorelements 12 with appropriate connections at one or both ends. On theother face of the thin dielectric film 10, by a similar technique, thereis provided a pattern of electrically conducting material forming asecond series of parallel conductor elements 14 with appropriateconnections at one or both ends which are orthogonal to, but not inelectrical contact with the first series. The first and second series ofconductor elements thus form a plurality of intersections 20.Appropriate material for these conductor elements 12, 14 is, forexample, silver-based conducting ink. If the conductor elements are tobe of low visibility where the touchpad is being used in front of adisplay system, then indium oxide is an appropriate material.

In other embodiments, the first and second series of conductor elementsneed not be parallel, nor is it necessary for the first and secondseries of conductor elements to be mutually orthogonal. The secondseries of conductor elements may be deposited onto a second thindielectric film, the second film being superimposed on the firstdielectric film in order to achieve similar effect of separation of thefirst and second series of conductor elements by a dielectric layer.

It is also possible to effect the superposition of the conductingelements in other ways. For example, in FIG. 2b the first series ofconductor elements 12 may be deposited onto the thin dielectric film 10and an insulating layer 13 deposited thereupon. The second series ofconducting elements 14 may then be deposited over the insulating layer.Thus, the insulating layer 13 forms a membrane structure between thefirst and second series of conducting elements.

The insulating layer 13 need not, however, be continuous over the entiretouchpad surface: it is only necessary to insulate the intersections ofthe first and second series of conductor elements. In FIG. 2c, thisarrangement is shown, where small regions of insulating material 13' aredeposited over the first series of conducting elements 12 at theproposed intersection points. The second series of conductor elements 14may then be deposited. In this instance, the small regions of insulatingmaterial 13' in conjunction with the dielectric film 10 form a membranestructure separating the first and second series of conductor elements.

Connection to the conductor elements 12 and the conductor elements 14 ismade by further conducting elements 32, 34 respectively deposited and/ordefined in similar manner to conductor elements 12, 14. A connection tothe touchpad scanning system is made by connector 30 using anappropriate connection system.

In the embodiments of FIGS. 1 and 2a-2c, the width 16 of the conductorelements 12 and 14 is small compared with the inter-element spacing 18.If the conducting material being used to form conductor elements 12, 14is of low conductivity, an alternative pattern of conductor element maybe used as described later.

In another embodiment, the inter-element spacing 18 need not beidentical for each adjacent pair of conductor elements.

The sensitivity of the touchpad and its immunity to extraneousinterference has been found to be enhanced by the encapsulation of thedielectric film 10 and conducting elements 12, 14, 32, 34 in adielectric laminate 50, as shown in FIGS. 2a-2c The dielectric laminatemay be a plastic film, and can be formed using well known techniquessuch as heat sealing. This provides a constant dielectric environment inthe immediate proximity of the conductor elements, eliminates theinfluence of moisture which might otherwise be present on the conductorelements, and further improves the robustness of the apparatus.

High sensitivity to changes in capacitance of a conductor element orgroup of conductor elements caused by the proximity of a finger or otherobject is achieved by minimising the cross-coupled capacitance betweenthe conductor elements 12 and the conductor elements 14. This can beachieved in one embodiment by the use of highly conductive material(such as silver) and the forming of conductor elements which have a verynarrow width 16 when compared to the conductor spacing 18 as previouslydescribed, such that the capacitance of the intersections 20 is small.In the event that it is desirable that a lower conductivity material beused (e.g. indium oxide), or that the dimensions of the touchpad becomesufficiently large such that there is substantial resistance along aconductor element, then alternative patterns may be considered such asthose embodied in FIGS. 3a and 3b.

In FIG. 3a, where the conductor elements have a more substantial width22, at the intersections 20 the width 24 is greatly reduced.

In FIG. 3b, the conductor elements 12 and 14 maintain full width 22, butthe second conductor element 14 has a "window" area 28 which has noconductive material. This "window" allows the necessary capacitive linkto the first conductive element 12. The window area 28 need not becompletely open. As indicated by dotted line 29, an area of conductormaterial electrically isolated from the second conductor element 14 canin fact be left within the window 28 and still provide the necessarycapacitive link to the first conductor element 12.

The relative thicknesses of the conductor elements thus can be varied tosuit the conductivity of the material being used, the length of thetracks, and other constraining factors. It is noted that the smallerwidth tracks can result in better resolution and higher speed ofoperation, but use of the wider tracks can be acceptable for lowerresolution, less sensitive or slower requirements.

In another embodiment, the sensitivity to changes in capacitance causedby the proximity of a finger or other object to a large area touchpad isenhanced by connecting several conductor elements 12, 14 together ingroups as embodied in FIG. 4. This particular embodiment is preferredwhere the required positional resolution of a keystroke can becompromised in favour of an increased area of touchpad. This particularembodiment confers upon the apparatus the additional benefit that damageto one of the conductor elements 12 or 14 causing a break in thatelement does not affect the performance of the system, provided that theconnection of each group of elements at both ends is made, as shown inthe embodiment of FIG. 4. If a fine wire is used to detect a large areathen the wire should be zig-zagged over that area. The wire could bezig-zagged with 1/4-1/5th of an inch spacing.

If required, the conductor elements can be electrically connected toelements on the opposite face of the dielectric film 10 by the provisionof appropriately placed holes 36 in the dielectric film as shown inFIGS. 4 and 5, filled with conductive material through which, forexample, conductor element 12 is connected to conductor element 32 inorder that connector 30 is only required to make contact to one face ofthe dielectric film 10. Such a system may also be used to form"underpasses" for the conductor elements if required on particularlycomplex conductor patterns. These "underpasses" may be used to effectthe intersection points of the first series of conductor elements 12 andthe second series of conductor elements 14.

It is further noted that where conductor elements are used which havesignificant resistance along the length thereof, it is possible tominimise the impact this has by providing conductor elements 32,34 tocontact both ends of conductor elements 12,14 respectively. It isfurther possible to provide conductor elements 32,34 inhigh-conductivity material, and conductor elements in the lowerconductivity material, the elements being coupled together in knownmanner.

In a particular embodiment of this invention the required sensitivity ofthe system to changes in capacitance on any given element is enhanced byensuring that all of the conductor elements 14-1 . . . 14-n and 12-1 . .. 12-n are maintained at the same potential (for example groundpotential, or V_(supply) hereinafter referred to as "ground potential")except for the conductor element being sampled. The grounding of allconductor elements not being sampled greatly reduces the effect of straycapacitance from other parts of the touchpad on the element beingsampled, thus providing a more reliable measure of any capacitive changethat may have taken place on the conductor element being scanned.

An appropriate system for scanning keyboards, such as that described inEuropean Patent No. 0185671 is readily applicable with some modificationto this touchpad. In one particular embodiment as shown in FIG. 8, eachof conductor elements 12-1 to 12-4, and 14-1 to 14-3 is connected at oneend to a resistor 71 having a high value such as 100 kohms when comparedwith the impedance of the detection circuit, for example 10 kohms. (Theparticular values of resistance used are exemplary, and may besubstantially varied according to the configuration of the system.) Eachof the resistors is connected to, for example, ground potential. Theother end of each of conductor elements 12-1 to 12-4, and 14-1 to 14-3is connected in turn via analogue multiplexer 75 to Output line 72.

Where there is significant resistance along the length of the conductorelements 12 and 14, improvements in the performance of the detectionsystem can be achieved by the placing of the resistors 71 at theopposite sides of the conductor elements 12 and 14 to that shown in FIG.8. In other words, the resistors are placed at the multiplexer 75 of thetouchpad and connected to ground or an active ground as hereinafterdescribed.

Output line 72 is connected to the input of a capacitance controlledoscillator 85, the output of which is connected to a divide-by-n circuit90, which provides the data output on line 92. An indexing counter 80,clocked by a remote clock on line 82, is operative to control theanalogue multiplexer, and to reset capacitance controlled oscillator 85and divide-by-n circuit 90. A processing means, not shown, is operativeto receive the data from divide-by-n counter on line 92, and store it ina plurality of locations, each allocated to a particular one of theconductor elements 12 and 14. Divide-by-n circuit 90 and othercomponents such as indexing counter 80 could be provided by means of asuitable standard microprocessor.

The scanning system thus samples each conductor element in turnaccording to the analogue multiplexer sequence, and stores eachcapacitance value in memory. These values are compared with referencevalues from earlier scans, and with other capacitance values in the samescan from the other conductor elements in order to detect a keystroke.Keystrokes must be above a threshold value to be valid. By havingseveral threshold values it is possible to determine the pressure of keypress or distance that the finger is away from the key. This may beuseful, for example, when moving a cursor across a screen and thenmaking a selection by pressing harder on a selected point.

The remaining features of the scanning mechanism are well described inthe cited document and will not be discussed further here.

Detected changes in capacitance on more than one conductor element inany one scanning sequence enables interpolation of a keystroke betweenthose conductor elements. In the two dimensional case, as shown in FIG.6, conductor element 14-3, and conductor element 14-4 cross conductorelements 12-1 and 12-2. A finger or other object at position 40 can bedetermined in the X-direction by the relative effect on the capacitanceof element 14-3 compared with element 14-4, and in the Y-direction bythe relative effect on the capacitance of element 12-1 compared withelement 12-2. In a typical application, conductor elements 12-1,12-2 . .. 12-n and 14-1,14-2 . . . 14-n will be sampled by the scanning systemin a sequential manner. Clearly, the same applies to the embodiment ofthe touchpad where the conductor elements are arranged in groups wherethe interpolation is made between the centre line 45 of each group ofconductor elements (FIG. 4).

It will be clear that the interpolation technique enables not only ananalogue representation of finger position on the touchpad to becreated, but also allows the use of an increased number of "boxes" orpredetermined key areas 60,61 over the number of element intersections,as indicated in FIG. 7. Such "boxes" or keypad areas could be arrangedin any number of configurations capable of being resolved by the system.

In an alternative embodiment, an active backplane may be incorporatedinto the touchscreen. For example, a plastic sheet upon which is coateda conductive film is laminated to the touchscreen. The output 72 isconnected to a unity gain non-inverting amplifier 73. The output of thisamplifier 73 is connected to the backplane conductor (not shown) whichmay cover all or part of the keypad. The backplane will be active sincethe voltage thereon will vary with the output on line 72.

The backplane could also extend to areas in front of the keypad to"shield" keys which are non-operative.

The backplane potential thus created could also be suitably connected tomaintain all conductors 12-n, 14-n which are not actively being sampledat a common potential equal to the active backplane potential ratherthan the common ground potential as previously described herein.

This can in certain uses of the touchscreen eliminate the requirementfor a completely conductive backplane film.

In FIG. 9 there is shown an example of an appropriate pattern ofelements for simulating a keypad configuration such as that normallyassociated with a typewriter keyboard. This particular embodimentcomprises the horizontal conducting elements 12, vertical conductingelements 14, conducting elements 32, 34 for connection to connector 30in similar manner to the embodiments described with reference to theFIGS. 1 and 2. The sensitivity of the system can be further enhanced bythe addition of further conducting elements 42, 44; elements 42 being inelectrical connection with conducting elements 12, and elements 44 beingin electrical connection with conducting elements 14, elements 42 beingpositioned such that centre of a box defined by the elements 42 issuperimposed on the centre of a box defined by elements 44, the elements12, 42 being on one face of the thin dielectric film 10, and theelements 14, 44 being on the other face of the thin dielectric film 10.The separate elements 42, 44 are indicated schematically to the side ofthe drawing of FIG. 9.

As indicated earlier, the first and second series of conductor elements12 and 14 need not be deposited on opposite faces of the same dielectricmembrane, but might be deposited on separate dielectric membranes, withsaid membranes being superimposed one on the other. This principle maybe extended to include a plurality of membranes, each having a separatepattern of conductor elements. These could, for example be PCB's(printed circuit boards) of known type.

The conductor elements 12,14,32 and 34 could be formed from fineconducting wires which would preferably be insulated by, for example, anenamel coating. The wires 12,14 could be allowed to touch atintersections 20, electrical contact being prevented by the insulatingcoating. Alternatively, the wires could be arranged on either side of asuitable membrane for mounting purposes. The wires may be from 10 to 25microns in diameter thereby being invisible to the naked eye when theinvention is used as a touchscreen.

In a further embodiment of the present invention, particularly anembodiment including, for example, a plurality of membranes havingconductor elements thereon, multiplexing techniques can be used.Duplicate sets of N small touchpads are arranged to form a largetouchpad array. This array is superimposed upon a larger touchpad with Mkeys (M could be equal to N). The position of a finger or other objectproximate to the first touchpad is interpretable by the system as a keystroke in any one of N possible positions. The second, larger gridpattern is used to determine which of the M possible duplicate key padshas been touched, enabling unambiguous determination of the position ofthe finger.

With reference to FIG. 10, there is a first grid pattern comprisingrepeating pattern of elements A to D and W to Z; that is to say that allA elements are electrically connected, all B elements are electricallyconnected, and so on. It is thus apparent that there will be four firstgrid horizontal connections A,B,C,D, and four first grid verticalconnections W,X,Y,Z. A second grid is placed directly over the firstgrid, the second grid having four horizontal elements with fourconnections a,b,c,d, and four vertical elements with four connectionsw,x,y,z. A finger placed at the position marked with a square on thefirst grid will be indicated by the first grid as interference withelements A and Z. Such interference would be the same for sixteenpositions on this grid, but the second grid will indicate interferencewith elements c and b, with c stronger than b, and interference withelements x and y, with x stronger than y. This enables uniquedetermination of the position of the interfering object. It is readilyapparent that 256 positions can thus be resolved by just 16 electricalconnections. If interpolation techniques are used, more than 256positions can be resolved.

If enamel coated wires are used then because these are insulated fromeach other a plurality of matrix wire arrangements can be placed on topof each other without any separating membrane.

More keys can be determined by duplicating or rearranging the order ofthe connections and thus determining the unique best and second bestvalues. For example, instead of A,B,C,D, as above the order D,A,B,C,D,B,could be used.

If D gives the best value in the above example and C is the second bestthen it is the second D that has been selected.

If D gives the best value and A is the second best then it is the firstD that has been selected.

This example can be accomplished in a linear or in a grid pattern toprovide more key positions and thus can be used in combination with theinterpolation techniques to provide even more key positions.

The grid patterns could be arranged as shown in FIG. 10 to be evenlyspaced, but equally each four by four pattern (A-D, W-Z) could bearranged, within reason, at any location on the touchpad surface or onanother surface thus providing 16 separate and distinct four by fourarrays. In the extreme case, each array could be constructed to be asingle key, the example shown thus providing 256 keys at remotelocations but not necessarily in a defined pattern.

In a very large keyboard, it may be required to sample the elements morequickly. This can readily be achieved by sampling several elements atonce. Thus for a 16×8 array, rows 1 and 9 might be sampledsimultaneously, 2 and 10, 3 and 11 and so on. If only one keypad is tobe operated at any one time, unambiguous determination can be obtained,since the rows will be sufficiently far apart that a stray signal willnot be possible.

The outputs may be fed to different inputs of a multiplexer circuit andthen to a common detector circuit. In the event that two valid outputsare received, a comparison would be made to determine the best signal,or a fault indicated requiring a further keystroke.

It will be readily apparent that the use of multiple layers ofdielectric membranes could readily be scanned by several detectorcircuits in communication with one another.

It is further noted that the use of a first series of thin conductorelements as described herein may effectively be used to form discretepad areas as, for example element 42 in FIG. 9 each with a separateconnector line to the scanning mechanism, and without the use of asecond series of conductor elements.

What is claimed is:
 1. A touchpad comprising an electrically insulatingmembrane (10), a first series of spaced apart wires (12) disposed on afirst face of said membrane (10) and a second series of spaced apartwires (14) proximal to said membrane, there being no electrical contactbetween said first and second series of spaced apart wires (12, 14),each wire of said first and second series of spaced apart wires beingsensitive to the proximity of a finger to modify the capacitance of thewire in proximity to the finger to detect the presence of the fingerpositioned in proximity to that wire each wire of said first and secondseries of wires having a diameter in the range of about 10 microns toabout 25 microns.
 2. A touchpad comprising an electrically insulatingmembrane (10) with a first series of spaced apart conductors (12) on afirst face of said membrane (10) and a second series of spaced apartconductors (14) on or proximal thereto, in which there is no electricalcontact between said first and second series of conductors (12, 14),each conductor in said first and second series of conductors beingsensitive to the proximity of a finger to modify the capacitance of saidconductor to detect the presence of the finger positioned close to saidconductor, said first and second series of conductors (12, 14)comprising very fine wires, said wires being enamel coated and being inthe range of about 10 microns to about 25 microns in diameter and ascanning system operative to sample each one of the conductors in turnfrom both said first and second series of conductors (12, 14) in orderto measure and store a capacitance value associated with that respectiveconductor, said scanning system being operative to maintain allconductors of said first and second series of conductors (12-n, 14-n) ata common potential when said conductors are not actively being sampledby said scanning system.
 3. A touchpad comprising an electricallyinsulating membrane (10) with a first series of spaced apart wires (12)on a first face of said membrane (10) and a second series of spacedapart wires (14) proximal thereto, in which there is no electricalcontact between the first and second series of spaced apart wires (12,14), each wire of said first and second series of spaced apart wiresbeing sensitive to the proximity of a finger to modify the capacitanceof the wire to detect the presence of the finger positioned close tothat wire, said first and second series of wires having a diameter inthe range of about 10 microns to about 25 microns, including a scanningsystem operative to sample a said one of said wires in turn from bothsaid first and second series of spaced apart wires (12, 14) in order tomeasure and store a capacitance value associated with that respectivewire, wherein the scanning system is operative to maintain all wires(12-n, 14-n) at a common potential equal to the potential of the wirebeing sampled when said first and second series of spaced apart wiresare not actively being sampled by said scanning system.
 4. A touchpad asclaimed in claim 3 in which said scanning system comprises a multiplexer(75), individual inputs of which are respectively connected to a firstend of each respective wire operative to scan each wire said first andsecond series of spaced apart wires in a defined order such that onlyone wire is connected to the output of the multiplexer at any timeinstant and including voltage driving means an input of which isconnected to the output of said multiplexer and an output of saidvoltage driving means is connected to all wires of said first and secondseries of spaced apart wires (12-n, 14-n), said voltage driving meansthereby maintaining the voltage level on each wire of said first andsecond series of spaced apart wires (12-n, 14-n) at a common potential,which common potential is equal to the output voltage potential of thewire being sampled at that time instant.
 5. A touchpad as claimed inclaim 4 in which said voltage driving means comprises a unity gainamplifier, the output of which is held at the same voltage as the input,thereby maintaining the voltage level of all wires of said first andsecond series of spaced apart wires at the same potential.
 6. A touchpadas in claim 4 wherein each of the wires of said first and second seriesof spaced apart wires is formed from a plurality of electricallyconnected conducting elements.
 7. A touchpad comprising an electricallyinsulating membrane (10) with a first series of spaced apart wires (12)on a first face of said membrane (10) and a second series of spacedapart wires (14) proximal thereto, in which there is no electricalcontact between said first and second series of spaced apart wires (12,14), each wire in said first and second series of spaced apart wiresbeing sensitive to the proximity of a finger to modify the capacitanceof the wire to detect the presence of the finger positioned close tothat wire, each wire of said first and second series of wires having adiameter in the range of about 10 microns to about 25 microns includinga scanning system operative to sample a wire in turn from both saidfirst and second series of spaced apart wires (12, 14) in order tomeasure and store a capacitance value associated with that respectivewire, wherein said scanning system is operative to maintain all wires ofsaid first and second series of spaced apart wires (12-n, 14-n) at acommon potential equal to the potential of the wire being sampled whensaid first and second series of spaced apart wires are not activelybeing sampled by said scanning system and in which said scanning systemcomprises a multiplexer (75), individual inputs of which arerespectively connected to a first end of each respective wire operativeto scan each wire of said first and second series of spaced apart wiresin a defined order such that only one wire is connected to the output ofsaid multiplexer at any time instant and including voltage driving meansan input of which is connected to the output of said multiplexer and anoutput of said voltage driving means is connected to all wires of saidfirst and second series of spaced apart wires (12-n, 14-n), said voltagedriving means thereby maintaining the voltage level on each wire of saidfirst and second series of spaced apart wires (12-n, 14-n) at a commonpotential, which common potential is equal to the output voltagepotential of the wire being sampled at that time instant.
 8. A touchpaduseable as a touch screen comprising an electrically insulating membrane(10) with a first series of spaced apart conductors (12) on a face ofsaid membrane (10) and a second series of spaced apart conductors (14)on or proximal thereto, in which there is no electrical contact betweenthe first and second series of spaced apart conductors (12, 14), eachconductor of said first and second series of conductors having a widthin the range of about 10 microns to about 25 microns, each conductor insaid first and second series of spaced apart conductors being sensitiveto the proximity of a finger to modify the capacitance of conductor todetect the presence of the finger positioned close to that conductorcharacterized in that said first and second series of spaced apartconductors (12, 14) comprise very fine insulation coated electricallyconductive conductors.
 9. A touchpad as set forth in claim 8 in whichsaid conductors are enamel coated.
 10. touchpad as set forth in claim 8wherein said second series of spaced apart conductors (14) is attachedto a further electrically insulating membrane (13).
 11. A touchpad asset forth in claim 8 wherein said first and second series of spacedapart conductors (12, 14) are arranged to form a plurality ofintersections (20) between said first series of spaced apart conductorsand said second series of spaced apart conductors.
 12. A touchpad as setforth in claim 8 in which said touchpad includes an active backplanedevice.
 13. A touchpad as set forth in claim 8 wherein said secondseries of spaced apart conductors (14) is attached to said face of saidinsulating membrane (10), said first series of spaced apart conductors(12) being insulated from said second series of spaced apart conductors(14) by discrete regions of insulating material (13').
 14. A touchpad asset forth in claim 13 wherein said membrane and said insulating material(10, 13') and said first and second series of spaced apart conductors(12, 14) are laminated between dielectric films (50).
 15. A touchpad asset forth in claim 8 wherein said second series of spaced apartconductors (14) are formed on a further face of said membrane (10). 16.A touchpad as set forth in claim 15 wherein said second series of spacedapart conductors (14) is attached to a further electrically insulatingmembrane (13).
 17. A touchpad as set forth in claim 16 wherein saidmembrane and said further membrane including said first and secondseries of spaced apart conductors (12, 14) are superimposed one aboveanother and a plurality of scanning mechanisms for scanning said firstand second series of spaced apart conductors.
 18. A touchpad useable asa touch screen comprising an electrically insulating membrane (10) witha first series of spaced apart conductors (12) on a face of saidmembrane (10) and a second series of spaced apart conductors (14) on orproximal thereto, said second series of spaced apart conductors (14)being attached to said face of said insulating membrane (10), said firstseries of spaced apart conductors (12) being insulated from said secondseries of spaced apart conductors (14) by discrete regions of insulatingmaterial (13'), the conductors of said first series of spaced apartconductors intersecting at intersections with the conductors of saidsecond series of spaced apart conductors, each of the conductors of saidfirst and second series of spaced apart conductors having a first width(22) for the larger part of their length, and a second width (24) beingsubstantially smaller than the first width, which second width iscoincidental with each of said intersections (20), each conductor insaid first and second series of spaced apart conductors being sensitiveto the proximity of a finger to modify the capacitance of conductor todetect the presence of the finger positioned close to that conductorcharacterized in that said first and second series of spaced apartconductors (12, 14) comprise very fine insulation coated electricallyconductive conductors.
 19. A touchpad system including a touchpadcomprising an electrically insulating membrane (10) with a first seriesof spaced apart conductors (12) on a face of said membrane (10) and asecond series of spaced apart conductors (14) on or proximal thereto, inwhich there is no electrical contact between the first and second seriesof conductors (12, 14), each conductor of said first and second seriesof conductors having a width in the range of about 10 microns to about25 microns, each conductor of said first and second series of spacedapart conductors being sensitive to the proximity of a finger to modifythe capacitance of a conductor to detect the presence of the fingerpositioned close to that conductor characterized in that said first andsecond series of spaced apart conductors (12, 14) comprise very fineinsulation coated electrically conductive conductors including ascanning system operative to sample in turn each conductor from bothsaid first and second series of spaced apart conductors (12, 14) inorder to measure and store a capacitance value associated with thatrespective conductor.
 20. A touchpad system as set forth in claim 19wherein said scanning system is operative to maintain all conductors(12-n, 14-n) of said first and second series of spaced apart conductorsat a common potential when none of the conductors are being activelysampled by said scanning system.
 21. A touchpad system as set forth inclaim 19 wherein said scanning system is capable of determining theposition of the finger relative to two or more conductors of said firstand second series of spaced apart conductors by means for interpolatingthe degree of difference in capacitance of the associated conductors.22. A touchpad system as set forth in claim 20 wherein the surface areaof said touchpad is divided into a plurality of boxes (60, 61), thepresence of a finger positioned on any one box being distinguishable bysaid system from a finger positioned on any other box.
 23. A touchpadsystem including a touchpad useable as a touch screen comprising anelectrically insulating membrane (10) with a first series of spacedapart conductors (12) on a face of said membrane (10) and a secondseries of spaced apart conductors (14) on or proximal thereto, in whichthere is no electrical contact between the first and second series ofconductors (12, 14), each conductor of said first and second series ofspaced apart conductors being sensitive to the proximity of a finger tomodify the capacitance of a conductor to detect the presence of thefinger positioned close to that conductor characterized in that saidfirst and second series of spaced apart conductors (12, 14) comprisevery fine insulation coated electrically conductive conductors, ascanning system operative to sample in turn each conductor from bothsaid first and second series of spaced apart conductors (12, 14) inorder to measure and store a capacitance value associated with thatrespective conductor and an active backplane device and wherein saidscanning system is operative to maintain all conductors of said firstand second series of spaced apart conductors at the same potential assaid active backplane device when the conductors are not being activelysampled by said scanning system.
 24. A touchpad useable as a touchscreen comprising an electrically insulating membrane (10) with a firstseries of spaced apart conductors (12) on a face of said membrane (10)and a second series of spaced apart conductors (14) on or proximalthereto, in which there is no electrical contact between the first andsecond series of spaced apart conductors (12, 14), each conductor insaid first and second series of spaced apart conductors being sensitiveto the proximity of a finger to modify the capacitance of conductor todetect the presence of the finger positioned close to that conductorcharacterized in that said first and second series of spaced apartconductors (12, 14) comprise very fine insulation coated electricallyconductive conductors oriented in a zig-zag manner.
 25. A multiple inputproximity detector including at least one touchpad useable as a touchscreen comprising an electrically insulating membrane (10) with a firstseries of spaced apart conductors (12) being disposed on a face of saidmembrane (10) and a second series of spaced apart conductors (14) on orproximal thereto, in which there is no electrical contact between thefirst and second series of spaced apart conductors (12, 14), eachconductor of said first and second series of conductors having a widthin the range of about 10 microns to about 25 microns each conductor insaid first and second series of spaced apart conductors being sensitiveto the proximity of a finger to modify the capacitance of the conductorin proximity to the finger to detect the presence of the fingercharacterized in that said first and second series of spaced apartconductors (12, 14) comprise very fine insulation coated electricallyconductive conductors and in which the juxtaposition of two or moreindependent capacitance inputs are used to detect the proximity of afinger, such detection only being accepted as valid when all theconductor capacitance inputs indicate a valid detection, where suchinputs may be juxtaposed next to a range of other inputs in uniquecombination such that when any one combination gives a true detectionfor all its individual inputs, a unique valid detection is determined.